FR2746562A1 - DIGITAL RADIOPHONIA SIGNAL TRANSMITTER - Google Patents

Abstract

The present invention relates to transmitters of digital radio signals more commonly known as DAB signals. In order not to degrade a DAB signal with a crest factor of 11 dB, amplifiers should be used to provide an average output power at least 11 dB below the maximum power they can supply in linear conditions. By performing a clipping (T) followed by a low pass filtering (Fb1, Fb2), it is possible, while maintaining an acceptable bit error rate, according to the standards currently in force, to reduce the crest factor to 7 dB. This gain of 11-7 = 4 dB on the crest factor makes it possible, to obtain a given output power, to use 2.5 times less amplifiers (4 dB cong 20 log 2.5). Application to the power amplification of DAB signals.

Description

The present invention relates to the transmission of radio signals

  digital more commonly known as DAB signals according to the acronym of their English name: Digital Audio Broadcast; it relates more particularly to the amplification of power of such signals in the

output stages of the transmitters.

  When amplifying a DAB signal by a module whose transfer characteristic must be used in its linear part, it must be ensured that the peak voltages of the DAB signal applied to this module are processed in the linear part of the characteristic. transfer. With a power amplifier, since the DAB signal applied to it generally has a difference of 11 dB between its mean and peak powers, it results that, to amplify the largest peaks in the linear part of the characteristic of the amplifier, the average output power must not exceed the maximum power of the amplifier minus 11 dB; for example a 100 W amplifier will deliver an amplified DAB signal whose average power should not exceed 100 W - 11 dB ie about 8W and this in order to pass power peaks to 100 W Knowing that power amplifiers are expensive and often represent more than half the price of a DAB power transmitter, it appears that the use of an amplifier to obtain an average power at most equal to 8% of its power. possibilities, constitutes a

  heavy handicap on the financial level.

  The object of the present invention is to reduce this disadvantage.

  This is obtained by a DAB signal processing, before power amplification, intended to reduce the difference between its peak power and its

average power.

  According to the invention there is provided a transmitter for digital radiophonic signals comprising, coupled in a series circuit, coding means for receiving a signal to be transmitted and in exchange for providing a pair of quadrature signals representing, in Cartesian coordinates, a complex input number, a modulator with four phase states, amplification means, characterized in that it comprises, inserted in series between the coding means and the amplification means, clipping means followed by low-pass filtering means, the clipping means performing a clipping to a predetermined value of the number module

entrance complex.

  The present invention will be better understood and others

  characteristics will appear from the following description and figures

  1, an emitter according to the known art, FIG. 2, an emitter according to the invention FIGS. 3 to 5, oscillograms of signals encountered during the study which led to the transition from the transmitter according to Figure 1 to the transmitter

according to Figure 2.

  In FIGS. 1 and 2, the corresponding elements are designated by

the same references.

  FIG. 1 represents a DAB signal transmitter according to the prior art in the case of an embodiment without passing through an intermediate frequency,

  the baseband signal directly modulates the transmission frequency.

  The transmitter according to FIG. 1 receives a useful audio signal on an input E of a DAB encoder, 1; the encoder 1 delivers, on two outputs, respectively two quadrature baseband digital signals, 1 and Q, sampled at 2.048 MHz in the example described. These two digital signals are converted into analog, respectively in two X converters 21, 22, then filtered in., Respectively in two filters sin x F1, F2. The two quadrature signals coming from the filters F1, F2 are applied to a modulator with four phase states, 3, called QPSK modulator in the following, according to the acronym of the Quadrature Phase Shift Keying words which mean modulation at four phase states. In modulator 3 the quadrature signals modulate a carrier signal provided by a not shown oscillator; the frequency of this carrier signal is

1472 MHz in the example described.

  The output signal of the modulator 3 is amplified in a preamplifier PO followed by five amplifiers P1 to P5 connected in parallel. An adder circuit, Ad, is the sum of the output signals

  amplifiers P1 to P5; its output is coupled through a pass filter

  Fs band at a transmitting antenna, S. In what follows it is question of bit error rate and

  Crest; the meaning of these terms is recalled below.

  The bit or BER error rate of a transmitted signal is the ratio of

  number of false bits of the signal on the total number of bits of the signal.

  The peak factor of a signal is equal to the ratio of its power

  peak at its average power; it is usually expressed in decibels.

  In the case of FIG. 1, a DAB signal exhibiting, for example, a peak factor of 11 dB necessitates a power reduction, back off in the English literature, of 11 dB during amplification in order to limit the distortions. nonlinear and retain the nature of the DAB signal. This means that by using, for example, an amplifier with a power of W defined at 1 dB of compression, the average output power can not exceed 100 W - 11 dB, ie approximately 8 W; the power drop is 92 W. By clipping the DAB signal at 6 dB relative to a peak power considered at 11 dB above the average power, it was found, in the example which will be described later, whereas the peak factor of the signal considered increases from 11 dB to 7 dB; with such a clipping the bit error rate or TEB undergoes a slight degradation but, despite this degradation, its

value remains in the standards.

  This clipped DAB signal requires a power drop of 7 dB during amplification in order to limit non-linear distortions and to preserve the nature of the DAB signal. This means that by using, for example, an amplifier with a power of 100 W set at 1 dB of compression, the average power output can reach up to 100 W - 7 dB, or about 20 W. The gain of 11-7 = 4 dB on the power recoil allows, compared to the previous case, to use 2.5 times less amplifiers (4 dB-10 log 2.5) to obtain an amplified signal of the same power and including out-of-band intermodulation products, intermodulation products or shoulders in the Anglo-Saxon literature, equivalent; this is true provided, in order not to have larger intermodulation products, to take certain specific precautions upstream of the amplifiers; these

  precautions will be given later.

  The clipping referred to above for the DAB signal can be carried out anywhere between the encoder and the output amplifiers in the DAB signal transmitter: just at the output of the encoder 1 or at the output digital-to-analog converters 21, 22 or at the output of modulator 3 or even at the mixer output in the case where the signal to be transmitted is modulated in intermediate frequency before being transposed, by mixing, into transmission frequency; depending on whether this clipping will be upstream or downstream of the modulator, it will therefore relate to a DAB signal made of two signals in quadrature or a single signal. It is the clipping at the output of the encoder 1 which has been chosen in the embodiment according to FIG.

  ease of implementation and the results it gives.

  This involves clipping on the DAB signal made of a pair of quadrature signals, I and Q, representing the Cartesian coordinates of a complex number (I, Q). For this purpose a clipping circuit which is, in fact, composed of calculation means performs the following operations: module calculation: p = 2 + Q2 - module calculation, p ', after clipping to a clipping value Pe: P '= Pe sip> Pe p, = p sip <Pe it should be noted that this clipping at the Pe value can be decibelised in decibels by 20 log (Pm / Pe), where Pm is the maximum value taken by the module of ( I, Q) computation of the complex signal cleaves (1 ', Q'), in Cartesian coordinates, by keeping to it the argument of the complex signal of origin (I, Q) I '= p'.sin [Arc tg (I / Q)] Q '= p'. cos [Arc tg (I / Q)] FIG. 2 is a diagram of a transmitter according to the invention which is distinguished from the transmitter according to FIG. 1 only by the introduction, between the encoder 1 and the digital converters - analog 21, 22, additional circuits, and the absence of amplifiers P3 to P5, that is to say

three out of five amplifiers.

  The signals I and Q delivered by the encoder 1 are not, in fact, directly clipped but are respectively applied to the inputs of two interpolation circuits M1, M2. A clipping circuit, T, has two inputs respectively connected to the outputs of the two interpolation circuits and two outputs respectively coupled to the inputs of the converters 21, 22 through low-pass filters Fbl, Fb2; thus the converters 21, 22 respectively receive signals 1 'and Q' which do not come from the circuit

  clipping after being filtered.

  The role of the interpolation circuits M1, M2 is to increase the useful bandwidth of the signal with respect to the signal coming from the coder in order to correct the spectrum aliasing phenomenon, aliasing in the literature

  Anglo-Saxon, caused by the clipping circuit.

  The role of the clipping circuit T is to perform the three operations discussed further to reduce to a predetermined value, Pe, the modules of those complex signals DAB which have a value

  greater than Pe, but without modifying the argument of these signals.

  The low-pass filters F1 and F2 are, in the example described, filters with 64 coefficients coded on 12 bits. Their role is to eliminate the products

  intermodulation caused by the clipping circuit.

  Figures 3, 4 and 5 are oscillograms obtained at the output of the modulator 3, that is to say after passing on the carrier signal at 1472 MHz; the scale of these oscillograms is 300 kHz per tile horizontally and 10 dB per tile vertically; the frequency of 1,472 MHz has been placed

in the middle of the oscillogram.

  The waveform of Figure 3 shows the spectrum of the signal at the output

  of the modulator 3 in a circuit configuration according to FIG. 1, that is to say

  say without clipping; as it appears in FIG. 3, the intermodulation products are at -56 dB. Moreover, the measurement of the peak factor of this modulator output signal gives a value of 11 dB. As for the measurements made by placing a receiver in series with the transmitter, they show that a bit error rate of 104 results in an increase of the carrier-to-noise ratio, C / N, of 0.1 dB with respect to the ratio C / N that

  one would get with a perfect transmitter and receiver.

  It should be noted that this way of presenting the C / N ratio according to the bit error rate, and not the opposite which would seem more logical, is due to the

  fact that it is a bit error rate value that is sought.

  The oscillogram of FIG. 4 shows the spectrum of the signal at the output of the modulator 3 in a circuit configuration which differs from that of FIG. 2 only in the absence of the low-pass filters Fbl, Fb2, replaced by shorts; in this configuration and for 6 dB clipping of the peak value, the intermodulation products are

-26 dB.

  The low-pass filtering carried out with the filters Fbl, Fb2 according to FIG. 2 makes it possible to reduce these intermodulation products to -55 dB, as it appears in FIG. 5. The measurement of the peak factor of this signal of the modulator 3, in the configuration according to Figure 2, gives a value of 7 dB; and the bit error rate, always 10-4, is obtained with a C / N ratio which goes to 0.4 dB, ie a degradation of 0.3 dB relative to

The transmitter according to FIG.

  This value of 0.4 dB is the maximum value currently allowed, according to the standards; it allows to go from a peak factor of 11 dB to a peak factor of 7 dB and thus, as explained above, reduces the number of output amplifiers from five to two for an average power transmitter output unchanged. This standard, which fixes the maximum degradation of the carrier-to-noise ratio, C / N, at 0.4 dB, is not definitive and the maximum degradation value is likely to be increased to 0.8 dB in the future. important will then be possible which will lead to a decrease in the crest factor higher than that of the example described using

Figures 2 and 5.

  It has been indicated in the foregoing that clipping could be performed anywhere between the encoder and the output amplifiers; in the case where the clipping is performed on the DAB signal while it is in analog, the interpolation circuits have no more reason to be; similarly, if the sampling in the encoder 1 is at a frequency sufficient to prevent the clipping of the spectrum after clipping is troublesome, the circuits

  Interpolation is no longer necessary.

Claims (3)

  A digital radio signal transmitter comprising, coupled in a serial arrangement, coding means (1, 21, 22) for receiving a signal to be transmitted and providing in exchange a pair of quadrature signals representing, in Cartesian coordinates, a complex number of input,   a modulator (3) with four phase states, amplification means (P0-   P5), characterized in that it comprises, inserted in series between the coding means (1, 21, 22) and the amplification means (P0-P5), clipping means (T) followed by means of low-pass filtering (FB1, Fb2), the clipping means performing a clipping at a predetermined value of the   module of the complex input number.   2. Transmitter according to claim 1, characterized in that the pair of quadrature signals supplied by the coding means (1, 21, 22) being in digital value, the clipping means (T) are connected to the coding means to receive this pair, calculate the module and the argument of the input complex number and output a pair of quadrature output signals representing, in Cartesian coordinates, a complex number of the same argument as the complex number of input and module equal to that of the complex input number clipped to a predetermined value. 3. Transmitter according to claim 2, characterized in that the coding means (1, 21, 22) comprise, at the output, means interpolation (M1, M2).